Metallocene catalyst compounds, those compounds comprising a metal center bound to at least one Cp (cyclopentadienyl and ligands isolobal to cyclopentadienyl), are useful catalysts in the production of polyethylenes. Metallocene-produced polyethylenes have many end-use applications such as films, impact resistant articles, cookware, etc. Each of these end use articles requires polyolefin resins with differing properties (density, flow properties, etc.). In the production of polyethylenes, it is often desirable to maintain certain useful resin properties while advantageously changing others. For example, it is often desirable to produce polyethylene copolymers of a particular density, yet having a wide range of melt flow properties: high melt flow for injection molding applications, and low melt flow for extrusion properties, for example. It would be desirable to improve the flow characteristics of certain polyethylenes while maintaining other properties such as density in a simple and predictable manner.
The flow characteristics of polyethylenes, often indicated by a measure of the melt index ratio (MIR, or I21/I2), are typically controlled by adjusting polymerization conditions such as comonomer flow, etc. However, this adjustment often leads to undesirable changes in other resin properties. In the case of polymerization processes that use metallocene catalysts, another possibility in controlling resin properties is in tailoring of the catalyst structure. While it is known that varying the structure of the metallocene catalyst influences the final polymer properties, the influences are often unpredictable, and the changes to the resin multivariate. For example, in adding substituents to a Cp ligand of a metallocene, two or more polymer properties may change, one to an advantage and one to a disadvantage for a given end use application. See, for example, METALORGANIC CATALYSTS FOR SYNTHESIS AND POLYMERIZATION 629-642 (Walter Kaminsky, ed., Springer 1999); B. Löfgren, New olefin copolymers synthesized by metallocenes, 3 RECENT RES. DEVEL. IN MACROMOL. RES. 117-131 (1998); H. H. Britzinger et al., Stereospecific olefin polymerization with chiral metallocene catalysts, 34 ANGEW. CHEM. INT. ED. ENGL. 1143-1170 (1995); P. C. Möhring et al., Homogeneous Group 4 metallocene Ziegler-Natta catalysts: the influence of cyclopentadienyl-ring substituents, 479 J. ORGANOMETALLIC CHEM. 1-29 (1994). However, what is not disclosed is a specific class of metallocenes simultaneously useful in producing polyethylenes of varying rheological properties and capable of maintaining low levels of comonomer incorporation, the later characteristic of which is an advantage in controlling, among other properties, the density of the polyethylene.
What would be desirable is a class of metallocenes that can be selected in such a way that the properties of the polyethylene catalyzed by the one or more metallocenes of the class can be predictably controlled. There are disclosures of metallocenes as being “good” comonomer incorporators such as in U.S. Pat. No. 6,410,659 B1, WO 98/28350 and WO 94/03506, which is consistent with the general observation of metallocenes being good comonomer incorporators relative to other olefin polymerization catalysts, such as pointed out by Karol et al. In METALOROANIC CATALYSTS FOR SYNTHESIS AND POLYMERIZATION at 632. Some metallocenes have been disclosed as being “poor” comonomer incorporators such as in WO 03/008465 A2 WO 03/008465, and U.S. Pat. No. 6,642,400 B2. These poor comonomer incorporating metallocenes are bridged bis-Cp compounds as in the former two disclosures, or otherwise highly complex systems as in the later patent. One disadvantage to those poor comonomer incorporating metallocenes is the relative complexity and hence cost of manufacturing such compounds. What would be desirable is a class of metallocenes that is simpler and less expensive to manufacture, yet provide a broader range of properties to the polyethylenes produced therefrom, preferably, the possibility of low comonomer incorporation.